So after doing a few measurements, i found out that my tank is a 160 l tank. HAHA who knew, but this is still too small for the ghost knife, so iv decided to get a quote for a tank measuring 1.5mx600x550, and this tank will hold +- 480 litres of water. Lets just hope i can get it soon.

Are you having a tank custom built? I ask because the knife fish needs a large tank not because it requires that much water, but because it gets to be so long. If you're having a tank built for it, you could save some liters (and probably a decent amount of money) to get a tank that's not as tall but is plenty long and wide, giving the fish plenty of space to move around.